1. Field of the Invention
The invention relates to methods for cladding superalloy components, such as service-degraded turbine blades and vanes, by laser beam welding. More particularly, the present invention methods weld one or more filler material layers to substrates along weld translation paths and regulate laser optical energy to compensate for localized substrate topology variations, facilitating uniform or deliberately modified energy transfer rates along the paths.
2. Description of the Prior Art
“Structural” repair of gas turbine or other superalloy components is commonly recognized as replacing damaged material with matching alloy material and achieving properties, such as strength, that are close to the original manufacture component specifications (e.g., at least seventy percent ultimate tensile strength of the original specification). For example, it is preferable to perform structural repairs on turbine blades that have experienced surface cracks, so that risk of further cracking is reduced, and the blades are restored to original material structural and dimensional specifications.
Repair of nickel and cobalt based superalloy material that is used to manufacture turbine components, such as turbine blades, is challenging, due to the metallurgic properties of the finished blade material. The finished turbine blade alloys are typically strengthened during post casting heat treatments, which render them difficult to perform subsequent structural welding. For example a superalloy having more than 6% aggregate aluminum or titanium content, such as CM247 ® superalloy, is more susceptible to strain age cracking when subjected to high-temperature welding than a lower aluminum-titanium content superalloy, such as X-750 ® superalloy.
Currently used welding processes for superalloy fabrication or repair generally involve substantial melting of the substrate adjoining the weld preparation, and complete melting of the welding rod or other filler material added. When a blade constructed of such a material is welded with filler metal of the same or similar alloy, the blade is susceptible to solidification (aka liquation) cracking within and proximate to the weld, and/or strain age (aka reheat) cracking during subsequent heat treatment processes intended to restore the superalloy original strength and other material properties comparable to a new component.
One known superalloy joining and repair method that attempts to melt superalloy filler material without thermally degrading the underlying superalloy substrate is laser beam welding, also known as laser beam micro cladding. Superalloy filler material (often powdered filler) compatible with or identical to the superalloy substrate material is pre-positioned on a substrate surface or sprayed on the surface during the cladding process. A “spot” area of focused laser optical energy generated by a fixed-optic laser (i.e., other than relative translation, laser and substrate have a fixed relative orientation during laser beam application) liquefies the filler material and heats the substrate surface sufficiently to facilitate good coalescence of the filler and substrate material, that subsequently solidify as a clad deposit layer on the substrate surface. Compared to other known traditional welding processes, laser beam micro-cladding is a lower heat input process, with relatively good control over melting of the substrate and rapid solidification that reduce propensity to cause previously-described solidification cracking. Lower heat input to the superalloy substrate during laser welding/cladding also minimizes residual stresses that otherwise would be susceptible to previously described post-weld heat treatment strain age cracking. While laser cladding welds have structural advantages over traditionally-formed welds, practical manufacturing and repair realities require larger cladding surface area and/or volume coverage than can be filled by any single pass applied cladding deposit.
To meet needs for adding volume to superalloy components, a laser-cladded deposit on a substrate can be formed from single- or two-dimensional arrays of adjoining solidified clad passes. Multiple laser-welded cladding passes and layers can be applied to build surface dimensional volume. Creating arrays of laser-clad deposits often results in microcracks and defects in the deposited material and underlying substrate in the heat affected zone material. Some defects are related to lack of fusion (LoF) that is common when there is insufficient localized laser optical energy heat input. As shown in FIG. 1, exemplary superalloy turbine blade 20 has a blade body 22. Original integrally-cast tip cap and squealer (not shown) are often damaged and eroded in service and require remnant cleanup removal to achieve a rework plane 23 at top of body 22. New separate tip cap 24 is machined from new matching cast material and placed on rework plane 23. The blade 20 then requires structural repair filling of the missing portion 28 along and to the right of the tip cap side 26 with a volume of superalloy filler, in order to restore the original structural dimensions of the tip cap and to fuse to both the tip cap side 26 and blade body 22 over rework plane 23. A two-dimensional filler weld array 30 of individually-applied laser clad deposits or passes 31-36 is formed by known laser cladding methods. The laser beam focus position and substrate surface are moved relative to each other after a single deposit (e.g., 31, etc.), formation to weld the next deposit (e.g., 32, etc.).
As noted in FIG. 1, the weld array 30 exhibits lack of fusion (LoF) at corners of every weld pass. The LoF is caused by combinations of one or more of localized variations in the blade 20′ substrate surface topology that require corresponding variations in laser optical energy transfer in order to maintain desired fusion, including: asymmetric heat sink properties; diminished power density; and surface reflectivity. Local surface topology 40 variations are shown schematically in FIG. 2. A previously applied solidified laser-clad deposit 50 has a curved surface 53 that is bounded by a high point 52 and an edge 54. The edge 54 is in contact with the substrate surface 42. The deposit 50 represents additional heat sink material that must be heated along with underlying substrate 40 when the next laser-clad deposit is formed in abutting relationship to create a continuous weld line. Additionally, the curved surface 53 spreads the laser beam energy transfer of the next adjoining deposit and reduces localized power density (e.g., watts) per unit area. Potentially the curved surface 53 also changes localized laser optical reflectivity, which may be compounded by non-uniform filler powder distribution, e.g., scattering away from the curved surface, adding additional reflectivity variance.
When the next laser cladding deposit 60 is applied in adjoining, overlapping relationship with existing deposit 50, a common uniformly applied power and/or filler powder distribution across the new laser focus zone would not apply sufficient localized fusion energy, causing a poorer than desired weld in the curved surface 53 portion within the overlapping region. An overall uniform increase in applied heat energy by the laser when forming deposit 60, in order to compensate for “worst case” lack of fusion in the curved surface portion 53 of the overlapping region, is more than required for good fusion of the substrate 40 to the right of the prior deposit edge 54. This results in over-melting, over-heating and over-stressing of the crack sensitive substrate material 40, which may unnecessarily instigate subsequent hot cracking and/or strain age cracking.
It is often desirable to build superalloy material dimensional volume in a newly fabricated or repaired service-degraded superalloy component, such as a turbine blade or vane. When known laser cladding methods are employed multiple pass layers are applied over previously deposited multiple pass layers to create the needed built up volume. Laser microcladding with fixed optics requires multiple passes to accomplish a typical repair buildup because the size of overall area to be repaired is large relative to the beam diameter at focus. Each pass overlap involves a challenge in ensuring that full fusion is achieved within each built-up layer and that full fusion is achieved with the previously-applied underlying layer. Typically in known fixed optic laser cladding processes weld solidification crystal alignment tends to shift from perpendicular to the substrate in the first few applied layers and then tends to shift at an increasingly skewed angle in subsequently applied clad layers. Microcracking often initiates upon such shifts in the inter-layer crystallographic orientation.
Thus, a need exists in the art for a laser welding method for cladding superalloy components, such as turbine vanes and blades, which facilities uniform welds with desired localized fusion along a translation path without degrading structural properties of the component substrate.
A need also exists in the art for a laser welding method for cladding multiple layers to superalloy components, such as turbine vanes and blades, that facilities formation of continuous welds along a translation path with desired localized fusion within each applied layer without degrading structural properties of the component substrate.
Another need exists in the art for a laser welding method for cladding superalloy components, such as turbine vanes and blades, that facilities uniform welds in multiple dimensions and/or layers with desired localized fusion within an applied layer without degrading structural properties of the component substrate or underlying previously applied cladding layers.
Yet another need exists in the art for a laser welding method for building up surfaces of superalloy components, such as turbine vanes and blades, by application of multiple laser cladding layers, that maintains epitaxial grain growth from the substrate through each successive layer, in order to reduce likelihood of microcracking that might otherwise occur with changes in clad inter-layer crystallographic orientation.